In information and communication technology, it is widely performed that signals are transmitted by using light as a carrier by an intensity modulation, a phase modulation, and so on. For such an optical transmission, an optical coupling device is required, in order to optically couple an optical semiconductor element such as a light-emitting element and a light-receiving element with an optical transmission line such as an optical fiber.
In recent years, as electrical parasitic capacitance of the light-emitting element, light-receiving element, and the like has become nonnegligible in such optical coupling devices, in accordance with a speeding up of optical signals to be transmitted, a size of a light-emitting area or a light-receiving area of elements tends to become smaller. For example, a diameter of a light-receiving area of a GaAs pin-type photodiode is miniaturized to approximately 50 μm to 60 μm so as to obtain a response of more than 10 Gbps. The miniaturization of the light-emitting area or light-receiving area of the element described above incurs deterioration of optical coupling efficiency, and problems such as deterioration of noise resistance characteristics, and shortening of a transmission distance may occur. It might be also necessary to insert a lens in an optical line to allow a large tolerance for a relative position between the optical semiconductor element and the optical fiber. However, using a lens increases the number of components, makes the position adjustment between components more difficult, and increases a cost of manufacturing.
Accordingly, an optical coupling device using a so-called direct optical coupling (butt joint) method has been developed, wherein the light-emitting area or the light-receiving area of the optical semiconductor element and the optical transmission line are directly coupled without using any lens.
As such a device, an optical module is known, in which electrical interconnection layers are formed on one principal surface of an optical transmission line holding member for holding optical fibers, and an optical semiconductor element is mounted on the principal surface by flip-chip bonding with the light-emitting areas or the light-receiving areas thereof facing the optical fibers.
However, since the optical module described above uses the same resins for protecting a portion connecting the optical semiconductor element and the electrical interconnection layers, and for filling a gap between the optical semiconductor element and the optical fibers, the resin material should have not only the characteristics required for the underfill resin but also the optical characteristics. This leads to a problem that it limits the flexibility in a selection of resin materials and manufacturing conditions, and therefore increases the cost of manufacture.